In many high power applications it is necessary to switch high voltage and high current levels. For example, a power utility company must frequently provide high potentials (e.g., many kV) to avoid having to provide unmanageably large current levels (e.g., hundreds or thousands of amperes). Further, if high potentials can be suitably switched, it is often possible to operate equipment directly from the alternating current high distribution lines, without the cost of providing a suitable transformer.
It is known in the art to use series-coupled thyristors to switch high voltage levels, especially at distribution line potentials. Thyristors are three terminal devices that have high voltage and high current ratings, but unfortunately do not turn-off in response to a signal applied to the device gate input terminal. Once turned on by a gate current input signal, thyristors remain latched in the on state until the signal at the output terminals is interrupted, thus permitting the thyristor to turn off. For this reason, thyristors are typically used in power converter applications where commutation occurs naturally. In a natural commutation application, if a second device is turned on before a first device is turned off, it is understood that current from the first device will automatically be diverted to the second device, whereupon the first device turns off. However, thyristors are not preferred in inverter or pulse width modulation applications because of the complex (and power consuming) auxiliary commutation circuitry required to forcibly commutate or properly turn off the devices.
More recently, gate turn-off thyristors ("GTOs") have been used in high potential switching applications where natural commutation does not occur. A GTO is a three terminal device somewhat similar to a conventional thyristor, except that the GTO can turn off in response to a typically large current signal at its gate input terminal. GTOs are commonly used in traction applications to implement direct current choppers, and GTOs have been proposed for use in static condenser applications and energy storage schemes. Like thyristors, GTOs have a relatively low intrinsic dV/dt limitation (e.g., 500 V/.mu.s) that requires external components to ensure that the rate of voltage change across the device after turn off does not exceed the limitation. Typically large snubber capacitors (e.g., 5 .mu.F) are used for this purpose, but unfortunately dissipate considerable energy, and limit the practical switching frequency of circuits using GTOs. Essentially, whatever energy is stored in the snubber capacitor after turning off the GTO is dissipated when the device is next turned on. Thus, for each off-on GTO cycle, one "quantum" of snubber energy is dissipated, and attempting to increase the GTO switching rate beyond say 180 Hz increases the snubber losses to an unacceptable level.
As such, GTOs are not useful for applications, where, for example, a high switching frequency is desired to reduce low order harmonic distortion of the voltage or current waveform. For the same reason, it is difficult to construct a fast pulse-width modulated inverter using GTOs for operation directly from high voltage alternating current lines, for example, to form an active filter and volt-ampere reactive ("var") compensator.
In high voltage, high-current switching applications, several devices (thyristors, GTOs, and the like) are series-coupled such that the voltage across any one device is less than the device breakdown voltage. Understandably it is important that the high voltage be dynamically shared among the series-coupled devices during turn-on and turn-off transitions.
While each series-coupled device may experience the same differential high voltage potential, devices coupled at the "top" of the series are at a higher absolute potential compared with devices coupled at the "bottom" of the series. Because of the higher absolute voltage potentials seen by the devices nearer the "top" of the series, it is often difficult to safely provide proper gate drive signals to the various devices. For the same reason, the generation of power to operate the various gate drive circuits can also be troublesome.
What is needed is an apparatus and method for safely switching high voltage at high current levels, especially in applications where natural commutation need not occur. Preferably such apparatus and method should operate using series-coupled three terminal devices, which devices are commercially available, self-contained, and modular. Further, such apparatus and method should not require snubbers with their excessive power dissipation and high frequency switching limitations.
In addition, there is a need for a means to modularize each series-coupled device and its associated circuitry, and to provide safe isolation from high voltage potentials seen by the various devices. Finally, the gate drive circuitry used to drive such devices preferably should derive operating power from the high voltage seen by each device, and dynamic voltage sharing should be ensured among the various series-coupled devices. The present invention discloses such an apparatus and method fulfilling these needs.